The present disclosure relates to a nitride semiconductor laser device, and more particularly, to a nitride semiconductor laser device in which a protective film is provided on a cavity facet.
In recent years, there is an increasing demand for a semiconductor laser device having higher power. A technique of imparting light resistance to such a semiconductor laser device is a key factor to secure the performance of the device.
For example, a blue-violet semiconductor laser device made of a group III-V nitride semiconductor material (AlxGayIn1−x−yN (note that 0≦x≦1, 0≦y≦1, 0≦x+y≦1)), such as gallium nitride (GaN) or the like, is a major device for achieving ultra-high density recording of an optical disc apparatus, and is currently reaching a level at which it can be put into practical use. The higher-power blue-violet semiconductor laser device not only allows high-speed writing of an optical disc, but also is essential to develop a new technical field, such as application to a laser display device or the like.
Recent infrared and red semiconductor laser devices made of a gallium arsenide (GaAs)-based semiconductor material have a facet window structure in which an impurity is selectively diffused into an active layer in the vicinity of a cavity facet. As a result, the forbidden band (band gap energy) of the active layer is expanded only in the vicinity of the facet, thereby reducing light absorption and heat generation at the cavity facet. Such devices achieve higher power and higher reliability.
On the other hand, for GaN-based semiconductor materials, since it is not easy to form a facet window structure by impurity diffusion, it is considerably important to provide a facet protective film (coating film) made of a dielectric material which is formed on the cavity facet.
The facet protective film may be formed in a semiconductor laser device by depositing aluminum nitride (AlN) using an Electron Cyclotron Resonance (ECR) sputtering device, which causes less damage on the facet (see, for example, Japanese Unexamined Patent Application Publication No. H09-194204 (hereinafter referred to as Document 1)).
In a GaN-based semiconductor laser device, a first protective film made of AlN contacting the facet, and a second protective film made of aluminum oxide (Al2O3) following the first protective film may be provided so as to suppress oxidation of the GaN facet, thereby preventing a deterioration in the facet (see, for example, Japanese Unexamined Patent Application Publication No. 2007-103814 (hereinafter referred to as Document 2)).
In a GaN-based semiconductor laser device, a first protective film made of AlN or aluminum oxynitride (AlON) contacting the cavity facet, and a second protective film which is a reflectance adjusting layer made of an oxide may be provided so as to maintain a high Catastrophic Optical Damage (COD) level and increase the reflectance of the light emitting facet (see, for example, Japanese Unexamined Patent Application Publication No. 2007-318088 (hereinafter referred to as Document 3)).
Moreover, in a GaN-based semiconductor laser device, the facet protective film may include a moisture absorption preventing layer (silicon nitride/titanium nitride) so as to suppress diffusion of moisture from ambient air, thereby preventing a deterioration in the facet (see, for example, Japanese Unexamined Patent Application Publication No. 2007-324193 (hereinafter referred to as Document 4)).